Motion complicates communications between wireless terminals and base stations since the motion normally produces changes in the distance between the wireless terminal and base station. The change in relative positions between a wireless terminal and base station can introduce transmission timing errors due to changes in the time required to communicate signals between a base station and the wireless terminal. Motion can also introduce frequency errors which are the result of Doppler shift which occurs due to motion and changes in relative distance between the base station and wireless terminal during the transmission process.
While some systems can tolerate some degree of motion, particularly when the motion is very slow, many systems fail as the relative motion between a base station and wireless terminal increases, e.g., to speeds commonly encountered in moving vehicles such as cars and trains. Airplanes, because they normally travel at even greater speeds then cars or trains, can be particularly problematic for a wireless terminal which is located on a moving airplane.
Where motion occurs, assuming the effect of the motion in terms of relative distance between the base station and wireless terminal can be predicted, transmission timing corrections can be made to compensate for the change in distance. In addition, if the change in distance is known and the rate of change is known, frequency corrections can be made to compensate for Doppler shift.
In one known system where a moving wireless terminal interacted with base stations located at fixed known locations, the wireless terminal was pre-programmed with base station location information. This pre-programmed known base station location information was then used in combination with wireless terminal position information obtained from an external global positioning system (GPS) device that was coupled to the wireless terminal via a cable. In the known system the GPS position information was compared to the pre-programmed known base station location information to determine the relative distance between the fixed known location of the base station which the wireless terminal communicated and the location of the wireless terminal as determined by the GPS device. Changes in the determined distance information were then used to estimate the rate of change in the distance between the base stations fixed location and the wireless terminal's location which changed over time due to motion. Transmission timing corrections were generated in the known system based the determined changes in distance while frequency corrections intended to compensate for Doppler shift were also based on the same information.
While the known system worked adequately for systems where the base stations had known fixed locations which were preprogrammed into the wireless terminal, the approach of relying on fixed known base station location information which is preprogrammed into wireless terminals has several disadvantages and is not suitable for many applications.
For example, the known approach of relying on preprogrammed base station location information is not suitable where the location of a base station is not know at the time of wireless terminal deployment and is therefore not available for pre-programming into the wireless terminal. Such location information may not be available because the base station was not deployed at the time the wireless terminal was deployed. Also, the base station with which a wireless terminal seeks to interact may not have been included in the list of base stations for which location information was preprogrammed into the wireless terminal since it was not foreseen that the wireless terminal would interact with the base station or the location information was not available to the issuer of the wireless terminal, e.g., because the base station corresponded to another service provider's network. Memory constraints may also be responsible for limiting the amount of base station location information preprogrammed into a wireless terminal, e.g., programming a wireless terminal with information indicating the location of all terrestrial base stations on the planet may not be practical.
The known system was not capable of taking into consideration base station motion and relied on base stations having a fixed predetermined known location to support successful communication. While preprogramming of base station location information may be satisfactory for some limited applications, e.g., where network roaming need not be supported, it may not be possible to program a wireless terminal with fixed base station location information because the base station may be in motion, e.g., mounted on a moving vehicle such as an airplane or non-geostationary satellite. In addition, at the time the wireless terminal is deployed, the location of a particular base station may not be known since the base station may be deployed after the wireless terminal enters service.
While fixed base station positions may be preprogrammed in some cases, base stations may be mobile, e.g., in the case of non-geostationary satellites and/or aircraft based base stations. Furthermore, in addition to the base station being mobile, the wireless terminal may also be in motion. Accordingly, there is a need for methods and apparatus which allow a wireless terminal to communicate with a mobile base station. In addition, it would be desirable if at least some, but not necessarily all methods and apparatus could support communication while both a base station and wireless terminal were in motion.